It is known that in modern computer printers and electronic typewriters, single paper sheets are fed by a rotating platen which cooperates with pressure rollers to convey the paper sheet to a zone where printing devices (generally a printing head reciprocating along guiding bars) perform the printing of a character row. To assure a good printing quality, it is required that the paper sheet to be kept adherent to the platen in the printing zone. To this purpose, pressure rollers are provided positioned along a generatrix of the rotating platen and downstream of the printing zone. These rollers press the paper against the platen. The rollers are idler mounted on a rod which is parallel to the platen axis and is provided with two ending arms, perpendicular to the rod and hinged to the printer side plates, so that the rod and the rollers mounted thereon can be alternatively moved well away from the platen and again brought in contact with the platen. In other words this structure (known as a "bail") can be opened and closed.
The rod is normally urged toward the platen by resilient means. For the correct insertion of a sheet in the printer, the bail must be opened so that the sheet may be wrapped around the platen up to the line of contact between the platen and the paper bail rollers. Then, the bail must be closed so that the pressure rollers keep the sheet in contact with the platen. In the past, bails were manually operated. In modern printers, automatic actuators are provided which include control electromagnets or mechanical devices actuated by motor members already present in the printer (such as the carriage motor or the paper feeding motor which rotates the platen). An example of such devices is described in European Patent Application EP-AS-0216394. All these devices are very complex and expensive. The present invention overcomes these disadvantages and provides an automatic paper bail actuator which is reliable, extremely simple and inexpensive.